1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers provided with dielectrics on resonators and to communication apparatuses provided with the dielectric filters or the dielectric duplexers.
2. Description of the Related Art
In general, for example, in dielectric filters provided with a plurality of dielectric resonators on dielectric blocks, unloaded Q factor (Qo) of the resonator decreases as the ambient temperature increases, and Qo increases as the ambient temperature decreases. This is due to the temperature dependency of the conducting loss of an inner conductor and an outer conductor provided on dielectric blocks. For example, a ten-degree increase in temperature causes an approximately two percent decrease in the conductivity of silver and copper. This decrease in the conductivity of electrodes results in a decrease in Qo. Thus, the insertion loss of dielectric filters increases as the temperature increases.
Also, in general, the insertion loss of band pass filters increases in a range from a pass band to an attenuation band at lower frequencies and a range from the pass band to an attenuation band at higher frequencies.
The frequency characteristics of the insertion loss (which are the transmission characteristics) required for the band pass filter are represented by a point determined by a frequency at which the insertion loss is a predetermined maximum value and by the predetermined maximum insertion loss (hereinafter, referred to as a critical point in a pass band).
The transmission characteristics are, however, shifted toward higher frequencies or lower frequencies in accordance with a change in the temperature, due to the temperature dependency of a resonance frequency determined by the dielectric constant of the dielectric.
As described above, the transmission characteristics of dielectric filters vary with temperature, under the influence of the temperature dependency of the conductivity of the electrodes and the temperature dependency of the dielectric constant of the dielectrics.
A dielectric filter and the like that achieve stable passband characteristics as much as possible over a wide temperature range is disclosed in Japanese Unexamined Patent Application Publication No. 2000-223908.
A dielectric duplexer disclosed in Japanese Unexamined Patent Application Publication No. 2000-223908 includes a dielectric filter having a lower-frequency pass band and a dielectric filter having a higher-frequency pass band. The dielectric filter having the lower-frequency pass band uses a dielectric with a positive temperature coefficient of the resonant frequency and the dielectric filter having the higher-frequency pass band uses a dielectric with a negative temperature coefficient of the resonant frequency. Thus, the increase in the insertion loss due to the increase in the temperature is suppressed. The insertion loss in the two dielectric filters for the higher frequencies and the lower frequencies are prevented from exceeding the value at the predetermined critical point in the pass band.
The transmission characteristics required for band pass filters are, however, also represented by a point determined by a frequency at which the attenuation is a predetermined minimum value and by the predetermined minimum attenuation (hereinafter, referred to as a critical point in an attenuation band), as well as the critical point in the pass band.
FIG. 11 shows the transmission characteristics of a known dielectric filter. Here, a point A represents a critical point in the pass band, and a point B represents a critical point in the attenuation band. If the temperature coefficient of the resonant frequency Tc of the dielectric is zero, the insertion loss increases by conducting loss of an inner conductor and an outer conductor at a high temperature. If the temperature coefficient of the resonant frequency Tc of the dielectric is a predetermined positive value, the entire pass band and an attenuation pole frequency are shifted towards higher frequencies at the high temperature.
Although determining the temperature coefficient of the resonant frequency of the dielectric suppresses the increase in the insertion loss near the critical point A in the pass band, the attenuation near the critical point B in the attenuation band decreases. This causes a problem when not only the insertion loss in the pass band but also the attenuation in the attenuation band is strictly determined.